Astronauts Rick Mastracchio and Clay Anderson are set for a third and final spacewalk Tuesday to complete the installation of a new ammonia coolant tank aboard the International Space Station. The astronauts also will mount a depleted tank in the shuttle Discovery's cargo bay for return to Earth and retrieve no-longer-needed debris shields.

Credit: NASA

The spacewalk is scheduled to begin at 3:11 a.m. EDT Tuesday, although Mastracchio and Anderson could begin earlier if their preparations go smoothly.

This will be the 143rd spacewalk devoted to station assembly and maintenance since construction began in 1998, the seventh so far this year and the third of three planned by Discovery's crew. Overall, it is the sixth spacewalk for both Mastracchio and Anderson.

For identification, Mastracchio, call sign EV-1, will be wearing a suit with red stripes around the legs. Anderson, EV-2, will be wearing an unmarked spacesuit.

The astronauts ran into problems during a spacewalk Sunday to mount a new ammonia tank on the space station. Getting a sticky bolt to engage took longer than expected and as a result, Anderson and Mastracchio were forced to defer hooking the tank up to the station's ammonia lines and retrieving space debris impact shields mounted on a storage deck.

"We've got two roll-over tasks that were originally planned as part of EVA-2 that are going to be added to the front end of EVA-3," said shuttle Flight Director Mike Sarafin. "Specifically, the ammonia tank quick-disconnect hook-up that we'll perform at the very beginning of the EVA."

The astronauts could have made those connections at the end of their second spacewalk but there would not have been enough time to decontaminate their suits in the event of even a small ammonia leak.

"So they'll pick up the fluid QD hook ups at the very beginning of this third spacewalk and that'll hook up the brand new ammonia tank to the International Space Station," Sarafin said. "Following that, they'll go get the orbital debris shields off of the external stowage platform No. 2 and temporarily stow those in the airlock. After they do that, they'll go on and essentially proceed with the remainder of EVA-3, in large part what was planned pre-flight."

After stowing a grapple fixture used to help move the new ammonia tank Mastracchio and Anderson will "get the old ammonia tank that's temporarily stowed on the outside of the (station) and return that to the shuttle's payload bay for return to Earth," Sarafin said.

Astronauts Stephanie Wilson and James Dutton, operating the station's robot arm from inside the Destiny module, will move the old ammonia tank to the back of Discovery's cargo bay where Mastracchio and Anderson will be waiting to bolt it in place on the same cargo carrier used by new coolant tank during launch.

After stowing a few tools, Anderson will retrieve an experiment carrier platform from the European Space Agency's Columbus lab module and Mastracchio will perform get-ahead preps for a spare S-band antenna assembly that will be installed on the station's upper Z1 truss during an upcoming shuttle mission.

Finally, Mastracchio will double-check the mounting of a radiator servicing fixture that was installed during the second spacewalk.

"We noticed it was wobbling a little bit," Sarafin said. "We're not sure if we actually torqued it down a little bit too fast because of the type of bolt that that uses or if they didn't tighten it down enough. So they're going to go out and troubleshoot that and make sure there's a good structural connection. That'll essentially wrap up our third and final spacewalk of the flight."

Because of the additional work added to the EVA-3 timeline, the astronauts will defer two other planned tasks to service an external camera and a hand-like attachment for the station's robot arm. Both tasks will be added to future spacewalks.

Inside the space station, meanwhile, the astronauts are pressing ahead with work to move supplies and equipment from the Leonardo cargo module into the lab complex. Russian engineers, meanwhile, are troubleshooting problems with the Elektron oxygen generator in the Russian segment of the station.

Space station Flight Director Ed Van Cise said the problem was not a pressing issue and that the station has plenty of supplies in the near term while the Elektron issue is resolved.

In a bit of good news, replacement of a component in the station's U.S water recycling system appears to have resolved a different problem and Van Cise said the hardware is now processing water normally.

"That replacement went very well," he said. "Last night, we started our first processing cycle (and) I'm happy to report the water processing system is completely functioning, everything is working very well. So we have fresh recycled drinking water for the crew, water to provide to the oxygen generator should we ever need it and we're in very good shape."

Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision I of the NASA television schedule; best viewed with fixed-width font):

EDT........DD...HH...MM...EVENT

10:21 PM...07...16...00...STS/ISS crew wakeup
10:56 PM...07...16...35...EVA-3: 14.7 psi repress/hygiene break
11:46 PM...07...17...25...EVA-3: Airlock depress to 10.2 psi

04/13
12:46 AM...07...18...25...MPLM transfers resume
01:36 AM...07...19...15...EVA-3: Spacesuit purge
01:51 AM...07...19...30...EVA-3: Spacesuit prebreathe
02:41 AM...07...20...20...EVA-3: Crew lock depressurization
03:11 AM...07...20...50...EVA-3: Spacesuits to battery power
03:16 AM...07...20...55...EVA-3: Airlock egress/setup
03:26 AM...07...21...05...EVA-3/EV-1: Ammonia tank (ATA) connections
03:26 AM...07...21...05...EVA-3/EV-2: Debris (MMOD) shield retrieval
03:56 AM...07...21...35...EVA-3/EV-1: MMOD shield retrieval
04:11 AM...07...21...50...EVA-3: Adjustable grapple fixture stow
04:41 AM...07...22...20...EVA-3: Payload bay setup
05:11 AM...07...22...50...EVA-3: ATA install on cargo carrier
05:56 AM...07...23...35...EVA-3EV-1: Grapple bar removal
05:56 AM...07...23...35...EVA-3/EV-2: Station arm (SSRMS) setup
06:26 AM...08...05...00...EVA-3/EV-2: LWAPA experiment platform retrieval
06:56 AM...08...35...00...EVA-3/EV-1: LWAPA install
07:26 AM...08...01...05...EVA-3/EV-1: Z1 S-band antenna preps
07:26 AM...08...01...05...EVA-3/EV-2: SSRMS cleanup
08:11 AM...08...01...50...EVA-3/EV-1: S1 radiator grapple fixture adjust
08:26 AM...08...02...05...EVA-3/EV-2: Get aheads
09:11 AM...08...02...50...EVA-3: Cleanup and ingress
09:41 AM...08...03...20...EVA-3: Airlock repressurization
09:56 AM...08...03...35...Spacesuit servicing
11:30 AM...08...05...09...Mission status briefing on NTV
12:01 PM...08...05...40...ISS daily planning conference
02:51 PM...08...08...30...ISS crew sleep begins
03:21 PM...08...09...00...STS crew sleep begins
04:00 PM...08...09...39...Daily video highlights reel on NTV
07:45 PM...08...13...24...Flight director's update
11:21 PM...08...17...00...Crew wakeup

Additional coverage for subscribers:
VIDEO: MONDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY

VIDEO: FLIGHT DAY 8 HIGHLIGHTS MOVIE PLAY
VIDEO: MONDAY AFTERNOON'S MISSION STATUS UPDATE PLAY
VIDEO: RUSSIAN PRESIDENT CALLS THE SPACE STATION PLAY
VIDEO: SPACE STATION CREW MARKS COSMONAUTICS DAY PLAY
VIDEO: ABC, MSNBC, FOX NEWS AND KUSA-TV INTERVIEWS PLAY
VIDEO: JAXA EVENT WITH JAPANESE ASTRONAUTS PLAY
VIDEO: SUNDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY

HIGH DEFINITION TV DAY 7: MEAL TIME IN ZVEZDA PLAY
HIGH DEFINITION TV DAY 7: GRUELING 7.5-HOUR EVA FINSHES PLAY
HIGH DEFINITION TV DAY 7: VENTURING OUTSIDE THE STATION PLAY
HIGH DEFINITION TV DAY 7: GETTING READY FOR ANOTHER EVA PLAY
HIGH DEFINITION TV DAY 7: OLD CREW SLEEP BUNK REMOVED PLAY
HIGH DEFINITION TV DAY 7: TRANSFERS AND MORE TRANSFERS PLAY

VIDEO: FLIGHT DAY 7 HIGHLIGHTS MOVIE PLAY
VIDEO: CREW'S HOME MOVIES FOR FLIGHT DAY 7 PLAY
VIDEO: SUNDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: FINALLY WINNING THE BATTLE WITH TANK BOLTS PLAY
VIDEO: NEW COOLANT TANK MANEUVERED INTO POSITION PLAY
VIDEO: DEPLETED AMMONIA TANK REMOVED FROM STATION PLAY
VIDEO: STEP-BY-STEP WALKTHROUGH OF EVA NO. 2 PLAY
VIDEO: PREVIEW OF FLIGHT DAY 7 ACTIVITIES PLAY
VIDEO: SATURDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY

HIGH DEFINITION TV DAY 6: LIFE OF A SHUTTLE ASTRONAUT PLAY
HIGH DEFINITION TV DAY 6: "BUCKET BRIGADE" ON THE STATION PLAY
HIGH DEFINITION TV DAY 6: WORF INSTALLED IN DESTINY LAB PLAY

VIDEO: FLIGHT DAY 6 HIGHLIGHTS MOVIE PLAY
VIDEO: CREW'S HOME MOVIES FOR FLIGHT DAY 6 PLAY
VIDEO: SATURDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: SCIENCE RACK INSTALLED BY JAPANESE ASTRONAUTS PLAY
VIDEO: NAVAL POSTGRADUATE SCHOOL EDUCATIONAL EVENT PLAY
VIDEO: CBS AND NEBRASKA MEDIA INTERVIEWS WITH CREW PLAY
VIDEO: SMOKE ALARM IN THE ZVEZDA SERVICE MODULE PLAY
VIDEO: PREVIEW OF FLIGHT DAY 6 ACTIVITIES PLAY

VIDEO: MISSION EXTENSION NEWS FROM FLIGHT DIRECTOR PLAY

HIGH DEFINITION TV DAY 5: SPACEWALKERS RETURN TO AIRLOCK PLAY
HIGH DEFINITION TV DAY 5: UNLOADING THE LEONARDO MODULE PLAY
HIGH DEFINITION TV DAY 5: STATION'S NEW EXERCISE MACHINE PLAY
HIGH DEFINITION TV DAY 5: OPERATORS OF THE ROBOTIC ARM PLAY
HIGH DEFINITION TV DAY 5: SPACEWALKERS FREE AMMONIA TANK PLAY

VIDEO: FLIGHT DAY 5 HIGHLIGHTS MOVIE PLAY
VIDEO: CREW'S HOME MOVIES FOR FLIGHT DAY 5 PLAY
VIDEO: STOWAGE RACKS MOVED INTO STATION PLAY
VIDEO: NEW LABORATORY FREEZER FOR KIBO PLAY
VIDEO: FRIDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: RATE GYRO ASSEMBLY REMOVED FROM STATION PLAY
VIDEO: NEW AMMONIA TANK STOWED IN TEMPORARY SPOT PLAY
VIDEO: ANCHORING FIXTURE ATTACHED TO AMMONIA TANK PLAY
VIDEO: EXPERIMENT RETRIEVED FROM JAPAN'S SCIENCE DECK PLAY
VIDEO: STATION'S ROBOTIC ARM LIFTS TANK FROM SHUTTLE PLAY
VIDEO: NEW AMMONIA COOLANT TANK UNBOLTED FROM CARRIER PLAY
VIDEO: SPACEWALKER PREPS AMMONIA TANK IN SHUTTLE BAY PLAY
VIDEO: THE START OF MISSION'S FIRST SPACEWALK PLAY
VIDEO: STEP-BY-STEP WALKTHROUGH OF EVA NO. 1 PLAY
VIDEO: PREVIEW OF FLIGHT DAY 5 ACTIVITIES PLAY
VIDEO: THURSDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY
VIDEO: THURSDAY AFTERNOON'S MANAGEMENT TEAM UPDATE PLAY

VIDEO: FLIGHT DAY 4 HIGHLIGHTS MOVIE PLAY
VIDEO: RADIO AND TV INTERVIEWS WITH CREW PLAY
VIDEO: THURSDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: LEONARDO MODULE ATTACHED TO SPACE STATION PLAY
VIDEO: STATION'S ARM GRAPPLES THE LEONARDO MODULE PLAY
VIDEO: NARRATED PREVIEW OF LEONARDO'S INSTALLATION PLAY
VIDEO: WEDNESDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY
VIDEO: WEDNESDAY AFTERNOON'S MANAGEMENT TEAM UPDATE PLAY

VIDEO: FLIGHT DAY 3 HIGHLIGHTS MOVIE PLAY
VIDEO: WEDNESDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: SHUTTLE CREW WELCOMED ABOARD THE STATION PLAY
VIDEO: POST-DOCKING OF THE SHUTTLE BAY AND TAIL PLAY
VIDEO: DISCOVERY DOCKS TO THE SPACE STATION PLAY
VIDEO: SHUTTLE PERFORMS 360-DEGREE BACKFLIP PLAY
VIDEO: PREVIEW OF RENDEZVOUS AND DOCKING ACTIVITIES PLAY
VIDEO: OBJECT LOST FROM SHUTTLE TAIL DURING LAUNCH PLAY
VIDEO: TUESDAY AFTERNOON'S MANAGEMENT TEAM UPDATE PLAY
VIDEO: THE FULL STS-131 LAUNCH EXPERIENCE PLAY

VIDEO: FLIGHT DAY 2 HIGHLIGHTS MOVIE PLAY
VIDEO: TUESDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: PREVIEW OF FLIGHT DAY 2 ACTIVITIES PLAY
VIDEO: NARRATED TOUR OF DISCOVERY'S PAYLOAD BAY PLAY
VIDEO: DESCRIPTION OF KU-BAND ANTENNA PROBLEM PLAY

VIDEO: FLIGHT DAY 1 HIGHLIGHTS MOVIE PLAY
VIDEO: SHUTTLE DISCOVERY BLASTS OFF! PLAY | HI-DEF
VIDEO: GO INSIDE MISSION CONTROL DURING LAUNCH PLAY
VIDEO: POST-LAUNCH NEWS CONFERENCE PLAY

VIDEO: LAUNCH REPLAY: VAB ROOF PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PRESS SITE PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PATRICK AFB PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD PERIMETER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: BEACH TRACKER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD CAMERA 070 PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD CAMERA 071 PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PLAYALINDA BEACH PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD FRONT CAMERA PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: KSC WEST TOWER PLAY | HI-DEF

VIDEO: ASTRONAUTS LEAVE CREW QUARTERS PLAY | HI-DEF
VIDEO: CREW FINISHES GETTING SUITED UP PLAY | HI-DEF

VIDEO: NARRATED REVIEW OF SHUTTLE'S PREPARATIONS PLAY | HI-DEF
VIDEO: NARRATED REVIEW OF PAYLOADS' PREPARATIONS PLAY | HI-DEF
VIDEO: PREPARING AN EXTERNAL TANK FOR LAUNCH PLAY | HI-DEF

VIDEO: TIME-LAPSE OF GANTRY ROLLING BACK FOR LAUNCH PLAY
VIDEO: LAUNCH PAD'S SERVICE TOWER RETRACTED PLAY | HI-DEF
VIDEO: PAYLOAD BAY DOORS CLOSED FOR LAUNCH PLAY | HI-DEF
VIDEO: ASTRONAUTS INSPECT THE PAYLOAD BAY PLAY | HI-DEF

VIDEO: STS-131 MISSION PREVIEW MOVIE PLAY | HI-DEF
VIDEO: PRE-LAUNCH INTERVIEW WITH COMMANDER POINDEXTER PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH PILOT JIM DUTTON PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH RICK MASTRACCHIOPLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH METCALF-LINDENBURGER PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH STEPHANIE WILSON PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH NAOKO YAMAZAKI PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH CLAY ANDERSON PLAY

VIDEO: PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: COUNTDOWN STATUS AND WEATHER OUTLOOK PLAY
VIDEO: PREVIEW OF DISCOVERY'S LAUNCH COUNTDOWN PLAY
VIDEO: ASTRONAUTS ARRIVE FOR LAUNCH PLAY | HI-DEF

VIDEO: FULL FLIGHT READINESS REVIEW NEWS BRIEFING PLAY
VIDEO: RECAP OF THE FLIGHT READINESS REVIEW PLAY | HI-DEF
VIDEO: HELIUM VALVE NO CONSTRAINT TO LAUNCH PLAY | HI-DEF
VIDEO: MANAGERS ASSESS ISSUES BEFORE FLIGHT PLAY | HI-DEF
VIDEO: UPDATE ON PRE-LAUNCH PREPS AT PAD 39A PLAY | HI-DEF

VIDEO: SPACEWALKING SUITS LOADED ABOARD PLAY | HI-DEF
VIDEO: INTERVIEW WITH STS-131 PAYLOAD MANAGER PLAY | HI-DEF
VIDEO: INTERVIEW WITH BOEING PAYLOAD MANAGER PLAY | HI-DEF
VIDEO: PAYLOADS DELIVERED TO LAUNCH PAD 39A PLAY | HI-DEF
VIDEO: TRANSPORT CANISTER ROTATED VERTICALLY PLAY | HI-DEF
VIDEO: LEONARDO PLACED INTO THE TRANSPORTER PLAY | HI-DEF
VIDEO: STATION'S NEW AMMONIA COOLANT TANK PLAY | HI-DEF
VIDEO: LEONARDO HATCH CLOSED FOR FLIGHT PLAY | HI-DEF
VIDEO: FILLING UP ONE OF THE SUPPLY RACKS PLAY | HI-DEF

VIDEO: CAN THE SHUTTLE PROGRAM AVOID LOOMING RETIREMENT? PLAY
VIDEO: WHAT ABOUT ADDING ONE MORE SHUTTLE MISSION? PLAY
VIDEO: FULL BRIEFING BY SHUTTLE AND STATION OFFICIALS PLAY
VIDEO: THE STS-131 MISSION OVERVIEW PRESENTATIONS PLAY
VIDEO: PREVIEW BRIEFING ON MISSION'S SPACEWALKS PLAY
VIDEO: THE ASTRONAUTS' PRE-FLIGHT NEWS BRIEFING PLAY

VIDEO: SHUTTLE EVACUATION PRACTICE PLAY | HI-DEF
VIDEO: CREW MODULE HATCH IS CLOSED PLAY | HI-DEF
VIDEO: ASTRONAUTS BOARD DISCOVERY PLAY | HI-DEF
VIDEO: CREW BRIEFED ON EMERGENCY PROCEDURES PLAY | HI-DEF
VIDEO: TEST-DRIVING AN EMERGENCY ARMORED TANK PLAY | HI-DEF
VIDEO: NIGHTTIME APPROACHES IN TRAINING AIRCRAFT PLAY | HI-DEF
VIDEO: ASTRONAUTS CHAT WITH REPORTERS AT PAD 39A PLAY
VIDEO: CREW ARRIVES FOR PRACTICE COUNTDOWN PLAY | HI-DEF

VIDEO: GANTRY PLACED AROUND DISCOVERY PLAY | HI-DEF
VIDEO: DISCOVERY REACHES PAD 39A PLAY | HI-DEF
VIDEO: OVERNIGHT ROLLOUT BEGINS PLAY | HI-DEF
VIDEO: SHUTTLE HOISTED FOR ATTACHMENT TO TANK PLAY | HI-DEF
VIDEO: CRANE ROTATES THE ORBITER VERTICALLY PLAY | HI-DEF
VIDEO: DISCOVERY MOVES TO ASSEMBLY BUILDING PLAY | HI-DEF

VIDEO: ASTRONAUTS VISIT THEIR SPACECRAFT PLAY | HI-DEF
VIDEO: CREW GOES INSIDE LEONARDO MODULE PLAY | HI-DEF

VIDEO: EXTERNAL TANK ATTACHED TO BOOSTERS PLAY | HI-DEF
VIDEO: FUEL TANK LIFTED INTO CHECKOUT CELL PLAY | HI-DEF
VIDEO: EXTERNAL TANK ARRIVES AT SPACEPORT PLAY | HI-DEF

VIDEO: DISCOVERY'S NOSE POD ATTACHED PLAY | HI-DEF
VIDEO: INSTALLING DISCOVERY'S MAIN ENGINES PLAY | HI-DEF
VIDEO: KSC'S SHUTTLE MAIN ENGINE SHOP PLAY | HI-DEF
VIDEO: GASEOUS NITROGEN TANK REMOVED PLAY | HI-DEF
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